1. Field of the Invention
This invention relates to high speed semiconductor devices, and more particularly to a heterojunction transistor-like structure utilizing tunneling as the primary mechanism for charge transfer through the base region of the device.
2. Description of the Prior Art
Conventional semiconductor transistors operate on the principle of transfer of charge carriers from the emitter region of the transistor to the collector region of the transistor, where the charge carriers are injected into the base region and diffuse to the collector. The injected carriers are minority carriers when they are in the base region so that, in the case of a p-type base region, the injected carriers are electrons while, in the case of a n-type base region, the injected carriers are holes. In order for such a device to operate, these minority carriers in the base region must traverse the entire thickness of the base having opposite conductivity type and be collected on the other side at a collector electrode. The transport of these carriers is determined by their diffusion and by drift. Diffusion is both inherently slow and dependent on temperature, and is generally faster at higher temperatures and slower at lower temperatures. The transit time for the carriers in passage across the base region in an inherent limitation on the high frequency operation of these devices, when used as amplifiers, oscillators, or storage devices and minority carrier storage effects in the base region adversely affect operation of transistors as switches.
In the conventional semiconductor transistor, the frequency response is affected by the equivalent base resistance of the device. This in turn is determined by the resistivity of the semiconductive material and the thickness and cross-sectional area of the base region. In the prior art, various attempts have been made to reduce the base resistance and thickness of the base region. For example, the base region has been heavily doped to lower its resistivity, but this can produce an undesirable lowering of the lifetime of the minority carriers in the base region and a corresponding decrease in the transport factor .alpha. of the transistor, which results in low emitter-collector transfer efficiency. Additionally, the doping level n.sub.e of the emitter has to be greater than the doping level n.sub.b of the base in order to have transistor action. This in turn means that the emitter capacitance C.sub.e will be increased which will adversely affect the high frequency response of the transistor.
In conventional transistors using homojunctions, it is impossible to have band edge discontinuities between the emitter, base, and collector regions. Thus, the emitter has to be more highly doped than the base in order that the injected charge carriers from the emitter to the base are greater in number than the reverse injected charge carriers from the base to the emitter. As will be appreciated later, the present improved semiconductor device uses heterojunctions having band edge discontinuities, the valence band edge discontinuity .DELTA.E.sub.v preventing reverse injection of carriers from the base to the emitter, while the conduction band edge discontinuity .DELTA.E.sub.c provides a tunneling barrier for charge carriers moving from the emitter to the collector. Thus, the present device does not have to have its emitter doping be larger than its base doping and in fact the reverse is true. This significantly improves the device characteristics on the emitter side so that base resistance can be kept low (by heavy doping) while emitter capacitance is also very low.
While attempts have been made to modify the geometry and doping of conventional semiconductor transistors to make them operate at higher frequencies, the prior art has also attempted to avoid the mechanism of diffusion as the primary charge transfer mechanism in such devices. Two examples of attempts to provide transistors which do not operate on the principle of diffusion of minority carriers through the base are illustrated in U.S. Pat. Nos. 3,225,272 and 3,358,158. In the former of these patents, two tunnel junctions are provided between heavily doped semiconductor materials with the spacing between the junctions sufficiently small that tunnel current from the emitter can proceed through the base region and across the collector junction. Thus, this is a triode having three heavily doped regions.
In this patent, good operability is questionable since the energy band diagrams show available energy states for electrons in the base region. If this is so, such electrons will stay in the base region and recombine therein with holes, rather than ever reaching the collector.
Notwithstanding the foregoing, U.S. Pat. No. 3,225,272 shows a device which suffers from a lack of integrity of the base-emitter junction and therefore has large inverse currents between the emitter and the base. Further, the emitter capacitance C.sub.e will be large since the space charge region on the heavily doped emitter side of the base-emitter junction is very small. There will be a high injection current in this device which will limit the probability of tunneling current and a high reverse injection will occur, resulting in small gain. On the collector side of that transistor, the integrity of the base-collector junction is doubtful due to the high doping of the base and collector regions. Also, there will be a large collector capacitance C.sub.c and extremely low breakdown voltages for the base-collector junction.
In U.S. Pat. No. 3,358,158, the collector is either degenerately or non-degenerately doped. If it is degenerately doped, the problems with respect to the tunneling transistor of the other patent occur. In the case of a non-degenerately doped collector, only a partial solution to the problem of low voltage breakdown of the base-collector junction will be obtained. However, this would create in the collector a relatively thick space charge region through which carriers must tunnel. The tunneling current might be either impeded completely or adversely increased when a collector voltage is applied. Thus, these prior art patents, which attempt to provide tunneling base transistors, still have many of the disadvantages inherent in conventional transistors using minority carrier diffusion across the base region.
In order to overcome the problems associated with prior art transistors and particularly those which attempt to utilize tunneling through the base region, applicants have provided a novel tunneling base transistor-like device which has many important advantages in addition to that of high speed operation. As will be more apparent later, the present device has both high input and output impedance, minimal leakage currents even though the base resistance is very low, very small emitter and collector capacitances, minimal electron-hole recombination, high transfer efficiency of charge carriers between the emitter and collector, and the absence of minority carrier storage effects in the base region. Thus, applicants have provided a tunneling base transistor-like device which can operate at extremely high speeds approaching those of a Josephson tunneling device, but which is easily operable at those speeds at room temperatures. Since the tunneling phenomenon is not itself temperature dependent, the only dependency on temperature will be the need to provide charge carriers of sufficient energy for tunneling across the base region in order to maintain the output current level of the device at low temperatures. This is easily achieved by increasing the emitter bias at very low temperatures of operation.
Accordingly, it is a primary object of the present invention to provide an improved high speed semiconductor device in which the dominant charge transfer mechanism is tunneling.
It is another object of this invention to provide a semiconductor tunneling base device having both high input impedance and high output impedance.
It is another object of the present invention to provide a device in which low leakage currents result even though the resistance of the base region is very small due to heavy base doping.
It is still another object of the present invention to provide a device which utilizes tunneling as the dominant charge transfer mechanism, but which minimizes electron-hole recombination in the base, emitter, and collector regions of the transistor.
It is a further object of the present invention to provide a novel transistor-like device in which there is negligible base current due to either recombination of charge carriers in the base and emitter or reverse injected current from the base to the emitter.
It is another object of the present invention to produce an improved tunneling base transistor-like device having a high transmission coefficient leading to high efficiency of charge transfer from the emitter to the collector.
It is yet another object of the present invention to provide an improved tunneling base semiconductor device in which there are no minority carrier storage effects in the base region.
It is another object of the present invention to provide an improved semiconductor tunneling base transistor-like structure which can be used as an amplifier, an oscillator, or as a switching element.
It is another object of the present invention to provide an extremely high speed, improved tunneling base device which can be fabricated by conventional techniques to produce individual devices or arrays of such devices having reproducibly uniform operating characteristics.
It is a still further object of the present invention to produce a tunneling base semiconductor device having very small emitter capacitance and collector capacitance.